The kidney regulates the volume and composition of body fluids. During embryonic development nephrons, the functional units of the kidney, form from intermediate mesoderm after it is induced to differentiate by ureteric bud contact (71). The goal of this project is to determine the role of Wnt genes in nephrogenesis and the mechanisms guiding ureteric bud formation and growth. Members of the Wnt gene family have been implicated in developmental regulation and Wnt-1 is expressed in embryonic spinal cord, a potent inducer of nephron formation in vitro (55,79). We demonstrate that fibroblasts expressing exogenous Wnt-1 trigger isolated intermediate mesoderm to differentiate into nephrons (32). Several members of the Wnt gene family are expressed in kidney and we will test whether any of these genes confer nephron-inducing activity to cultured fibroblasts (32,79,83). Wnt gene-products are secreted matrix-binding glycoproteins and it is likely that the nephron inducing protein(s) secreted by the ureteric bud are matrix associated (5,71). Fibroblast-free Wnt + extracellular matrices will be prepared and tested for nephron inducing activity in vitro. All models describing ureteric bud formation are based on the poorly characterized mechanisms mediating nephric duct growth (58). The temporal and spatial distribution of Wnt-11 mRNA raises the possibility that it plays a role in nephric duct morphogenesis. We will determine if persistent Wnt-11 expression perturbs this process. Using retroviral mediate gene transfer we will test whether the avian nephric duct and ureteric bud form exclusively by growth of the duct epithelium, or by growth of the duct and induced recruitment of cells from posterior intermediate mesoderm (48). This latter mechanism mediates nephric duct elongation during Xenopus development, and recent lineage analyses in our laboratory raise the possibility that the rat ureteric bud/renal collecting system grows in an similar manner (12). Lineage analyses testing the differentiated fate of isolated intermediate mesoderm will be strengthened using an additional histochemical marker to identify the ureteric bud/renal collecting system and an independent method of marking cells with a genomic tag. In addition, we will test whether RET, Sd, or Ld mutant intermediate mesoderm, like wild type, is competent to form the renal collecting system when co-cultured with homogeneously marked ureteric buds (40,45,74). Results of these experiments will test the hypothesis that renal collecting system morphogenesis in vivo is dependent on recruitment of cells from intermediate mesoderm.